Differences Between Medusa and Polyp Body Forms
The phylum Coelenterata comes under the Invertebrates. It is also known as Cnidaria. This is because they possess cnidoblast cells. These coelenterates have two types of body forms. They are the Polyp form and medusa zoology form. The medusa invertebrate body type is achieved by the process of metagenesis. This phylum has about 9000 species. The manubrium is just like a food pipe in these cnidarians. It connects the stomach of Coelenterata to the four parted mouth. Hydra and Aurelia come under this phylum. We will learn about the characteristics of this phylum and about the various body forms such as polyp and medusa zoology that are present in them. We will also learn more about the manubrium which is present in them.
General Characteristics
We studied above that this phylum is also known as Cnidaria. Some general characteristics features of this phylum are:
They inhabit aquatic lifestyle and are mostly present in the water and water bodies. Most of them are present in marine water but some of them are present in freshwater also. Some of these organisms live in solitary and some of them are present in colonies. The organisms that live independently are known as living in solitude. The other types of organisms form colonies and they are dependent on each other. These types are called colonial animals.
These organisms have radially symmetrical bodies. This means that their body can be divided into two equal halves when a plane passing through their central body divides them. These organisms have tissue level type of organization. They were the first organisms to exhibit tissue-level organization.
These organisms are diploblastic in nature. Epidermis and gastrodermis are formed from their germ layer. The nerve cells also occurred for the first time in these organisms. Their interstitial cells of the epidermis become specialized and form the stinging cells. Throughout their epidermis, their sensory cells are scattered.
Body Forms
The coelenterates or the cnidarians have two types of body forms. They are the polyp form and the medusa zoology form. These two body forms differ from each other in their structure and function. A polyp is a cylindrical form. This form remains attached to the substratum hence it is known as sessile form. The medusa invertebrate body type is an umbrella-shaped form. As it is not attached to anything therefore it is a free-swimming form. There is a phenomenon that takes place in the coelenterates. It is known as metagenesis. By this process, alternation of generation takes place. This means that the medusa invertebrate body type is changed to polyp forms and vice versa.
Reproduction in the polyp form is asexual whereas it is sexual reproduction in the medusa form. The polyp form undergoes budding which means that asexual reproduction happens and then it gives rise to medusae. The medusae then produce gametes and these gametes are the sperm and the ova. These two then participate in sexual reproduction and it gives rise to the polyp forms. The polyp form is sessile in nature and is present in hydra. All the other organisms having polyp form have hydra-like bodies. They have a cylindrical stalk. This stalk is attached to their mouth. They have tentacles that face upwards.
The medusa form on the other hand is a free-floating form. Their swimming structure is like that of the jellyfish. They have an upside-down body with a reduced stalk and they can swim easily. Their zygote is known as a planula. This planula swims, settles and grows into polyp forms. The hydra has no larval form. Both the sexual and asexual forms in coelenterates are diploid in nature. The haploid cells are gametes. Their skeleton is formed of calcium carbonate.
Digestive Tract
The coelenterates have a gastro-vascular cavity. This cavity opens up to the outside and forms the mouth. The mouth of the coelenterates is present on the hypostome. The hypostome is an elevation that is in the form of a cone. It is present on the free end of the body. The manubrium is known as their food pipe. It starts from the mouth and then gets attached to the stomach. The mouth is the only opening through which both the entry and the exit of the food and the waste material takes place. The gastro-vascular cavity is the central cavity and this cavity is named so because it helps in the digestion of food. It also helps in the circulation of materials.
Tentacles are present on the external body and these structures help them to catch food easily. The cnidoblast cells help in capturing the food. The cnidoblast cells are present in the gastro-vascular cavity. On the digested food, the gastric juices act. This digestion in their body occurs in the external cavity and this is thus known as extracellular digestion. By this digestion, the food is broken down into smaller fragments.
Hydra
Hydra is one of the main examples of this phylum. Some important features of hydra are:
They are present in the majority in cool, clean and stagnant water.
6-10 tentacles surround the mouth of the hydra. These tentacles are hollow in nature.
Daphnia and cyclops are the foods of hydra. The hydra cannot digest starch.
They are dioecious in nature. This means that they have separate sexes.
1-8 conical testes are present in the males.
The females bear 1-2 rounded ovaries.
The testis is present on the distal end and the ovaries are present on the proximal end.
The hydra moves by looping, gliding, and floating.
FAQs on Medusa Invertebrate Body Type Explained
1. What is the medusa body type found in invertebrates?
The medusa is one of the two principal body forms found in the Phylum Cnidaria. It is characterised by a free-swimming, gelatinous, bell-shaped or umbrella-shaped body. The mouth is typically located on the underside, at the end of a central stalk-like structure called the manubrium, and is surrounded by tentacles. This body form is most commonly associated with jellyfish.
2. What are the main structural components of a medusa?
The main structural components of a medusa include:
Bell (or Umbrella): The main body, which contracts for jet propulsion-based movement.
Manubrium: A tube-like structure hanging from the centre of the bell, which contains the mouth.
Mouth: The single opening for ingestion of food and ejection of waste.
Tentacles: Appendages extending from the margin of the bell, equipped with stinging cells (cnidocytes) for defence and capturing prey.
Gastrovascular Cavity: An internal cavity where digestion occurs.
3. How does the medusa body form differ from the polyp body form in Cnidarians?
The medusa and polyp are two distinct body forms in Cnidarians with several key differences:
Shape: A medusa has a bell or umbrella shape, while a polyp has a cylindrical, stalk-like shape.
Mobility: Medusae are generally motile (free-swimming), whereas polyps are sessile (attached to a substrate).
Orientation: In medusae, the mouth and tentacles face downwards. In polyps, the mouth and tentacles face upwards.
Reproduction: The medusa stage is typically associated with sexual reproduction, while the polyp stage is often associated with asexual reproduction (budding).
4. What is the primary function of the medusa stage in the life cycle of a cnidarian?
The primary function of the medusa stage is sexual reproduction and dispersal. Being motile, medusae can travel through water, which allows them to release gametes (sperm and eggs) over a wide area. This promotes genetic mixing and enables the species to colonise new habitats, which would not be possible for the sessile polyp stage.
5. Why is the free-swimming nature of the medusa form ecologically important for Cnidarians?
The free-swimming nature of the medusa is crucial for several ecological reasons. Firstly, it facilitates genetic diversity by allowing individuals from different colonies to interbreed. Secondly, it serves as the primary mechanism for species dispersal, helping cnidarians colonise new environments and expand their geographical range. Finally, mobility allows them to actively hunt for prey in the water column and escape localised threats or unfavourable conditions that could wipe out a stationary polyp colony.
6. Give an example of a common organism that demonstrates the medusa body type.
A classic example of an organism with a dominant medusa body type is the true jellyfish, such as Aurelia (moon jellyfish). In these organisms, the medusa is the large, visible, and free-swimming stage that most people recognise as a jellyfish. The polyp stage in their life cycle is typically small and inconspicuous.
7. Do all cnidarians that have a medusa stage also have a polyp stage? Explain the concept of metagenesis.
No, not all cnidarians exhibit both forms. The life cycle that alternates between an asexual polyp stage and a sexual medusa stage is called metagenesis (alternation of generations). A good example is Obelia, which has both distinct stages. However, some cnidarians have lost one of the stages during evolution. For instance:
Sea anemones and corals (Class Anthozoa) exist only in the polyp form and do not have a medusa stage.
True jellyfish (Class Scyphozoa) have a dominant and complex medusa stage, with the polyp stage being significantly reduced or short-lived.
